Carbazoles are heterocyclic compounds with fused benzene and pyridine ring systems. They have high thermal stability, good solubility and strong luminescence properties. Carbazoles can be functionalized with various substituents to tailor their electronic and optical properties. Among small molecule semiconductors, they are often used as luminescent materials in OLEDs and hole transport materials in OPVs. Their planar structure and excellent carrier transport properties make them attractive for use in high-performance organic electronic devices.